The objective of this research is to learn more about the mechanisms underlying both normal ocular motor control in normal humans and disordered eye movements in human and non-human primates, mechanisms providing plasticity, and strategies to restore normal function in patients with strabismus. The research strategy is to make quantitative measurements of oculomotor function in both normal volunteers and superior oblique palsy patients, perform adaptive manipulations and to use systems approaches to interpret findings. A novel haploscopic viewing device is used to create full-field cyclovertical disparity without errors in magnification (as would be expected if flat prisms were used). Next, we analyze static and dynamic eye movements in monkeys with superior oblique muscle palsy (SOP), investigating how ocular alignment changes with time. We also develop clinically useful diagnostic tests to stratify patients into different treatment categories. Finally, we dissect the neural circuitry and muscular mechanisms that provide plasticity to the ocular motor system. Specifically, we examine the role of global multiply-innervated fibers (MIF's), which have been previously overlooked because of their lack of fast twitch fibers, and their palisade endings in both proprioception and effecting static eye alignment. The focus is on mechanisms that maintain oculomotor accuracy with emphasis on binocularity and adaptive mechanisms. Particular attention will be given to the concept of abnormal cyclovertical vergence tonus and its effect on transient eye misalignment in normal subjects or as an etiology for primary cyclovertical strabismus (currently lumped with congenital superior oblique palsy). These results will provide new information about adaptive control of eye alignment relative to strabismus and provide the basis for non-surgical therapy in the future.